1. Field of the Invention
The present invention relates to a power converter such as an inverter device, and particularly to a power converter having a hybrid integrated circuit equipped with power semiconductor elements used in a power unit and control circuit elements used in a control unit, etc.
2. Description of the Related Art
FIG. 1 shows a circuit configuration of an inverter device which is one example of a power converter. Reference numeral 1 indicates a portion used as a semiconductor module in the invention. Reference numeral 2 indicates a power unit for converting power into another one. Reference numeral 3 indicates a control unit, which is integrated in the semiconductor module 1 as needed, of a control circuit of the power converter. A control circuit 4, which is provided outside the semiconductor module 1, performs the transfer of a control signal from and to the semiconductor module 1 through a control terminal 24.
Reference numerals 43 indicate input/output terminals for connecting power's input/output lines. Reference numerals 11c indicate power semiconductor elements such as diodes for rectifying three-phase alternating current power inputted from terminals R, S and T to dc power. The power semiconductor elements 11c are connected in three-phase bridge form to constitute a power rectifier. Reference numeral 42 indicates a ballast capacitor for smoothing the dc power outputted from the power rectifier to thereby obtain a dc voltage having less voltage pulsation. The ballast capacitor 42 is mounted outside the semiconductor module 1 and its electrodes are electrically connected to P and N for the dc power. Reference numerals 11a indicate switching elements, e.g., power semiconductor elements such as IGBT, for inversely converting the dc power obtained from the power rectifier to ac power having an arbitrary frequency again. Reference numerals 11b indicate power semiconductor elements used as feedback diodes for causing a feedback current fed from a load motor 5 to flow. The power semiconductor elements 11a and 11b constitute a power inverter connected in three-phase bridge. The power inverter receives signals for switching from the control unit 3 at terminals GU through GZ and inversely converts dc power to three-phase ac power. The reversely-converted three-phase ac power is outputted from terminals U, V and W of the input/output terminal 43. The motor 5 is connected to the terminals U, V and W and driven so as to be variable in speed.
FIG. 2 shows a specific example of the semiconductor module 1 which has heretofore been implemented. The semiconductor module 1 illustrated in the conventional example is one in which the power unit 2 and the control unit 3 are integrally formed in mixed form. FIG. 2 shows a structure sectional view thereof.
In the conventional structure, an insulated metal circuit board 19 typified by an aluminum insulated circuit board or the like considering a radiation property is applied as a metal base on which power semiconductor elements 11a and 11b are mounted. A resin insulating layer 20 formed by mixing a resin and a high -thermal conductive filer together is normally provided on the side of the mounting of the semiconductor parts 11a and 11b on the insulated metal circuit board 19. Each of foil-shaped laminated conductors 14 for fixedly securing and wiring parts for the power unit and the control unit is formed on the resin insulating layer 20.
Since a low heating part like an integrated circuit 23 of the control unit requires no radiation, it is placed inside the semiconductor module 1 by using, for example, a glass epoxy cheap printed circuit board 21a. Main terminals 17 (lead terminals connected to the input/output terminals 43) and control terminals 24 (lead terminals connected to the external control circuit 4) are embedded in a resin-molded outer package 33. Parts of the main terminals 17 and control terminals 24 guided inside the resin-molded outer package 33, the respective power semiconductor elements 11a and 11b, the laminated conductor 14 on the insulated metal circuit board 19, the laminated conductor 14 on the printed circuit board 21a, etc. are electrically connected to one another by metal wires 16. Further, the surfaces of the parts lying within the resin-molded outer package 33 are filled with a resin sealing agent 32 so that the respective built-in parts are covered therewith. Since the insulated metal circuit board 19 is expensive as compared with the printed circuit board 21a, a reduction in the occupied area of the insulated metal circuit board 19 would become a large factor for a cost reduction in the present semiconductor module. However, there is a tendency that if one attempts to bring a system into one package as a product, parts to be mounted therein increase and hence the size of the insulated metal circuit board 19 is also enlarged. Since the electrical connections between the main terminals 17 corresponding to the input/output unit of the power converter, the respective power semiconductor elements 11a and 11b, the laminated conductors 14 on the insulated metal circuit board 19, etc. are made by the metal wires 16 in addition to the above, the metal wires 16 need to be provided as metal wires constructed in plural form consistent with current capacity, thus leading to an increase in the pad area (area of each wire connecting portion) for each metal wire 16 and interference with a size and cost reduction.
On the other hand, a semiconductor module structure disclosed in Japanese Unexamined Patent Publication No. Hei 10-125826 as one proposed to avoid such a problem. A sectional view of this structure is shown in FIG. 3. A lead frame 13 formed integrally with each main terminal 17 acts the role of the laminated conductor 14 shown in FIG. 2. The power semiconductor elements 11a and 11b on a metal base 15 are all mounted on the lead frame 13. The lead frame 13 electrically connects between each main terminal 17 and the power semiconductor elements 11a and 11b. As to a base plate for the semiconductor module, a metal plate of aluminum or copper is applied as the metal base 15 in place of the insulated metal circuit board 19 shown in FIG. 2. The lead frame 13 and the printed circuit board 21a are adhered to the metal base 15 with a thin insulating adhesive sheet 18 interposed therebetween. Here, the insulating adhesive sheet 18 acts the role of the resin insulating layer 20 shown in FIG. 2 and contains a thermosetting resin such as an epoxy resin as a component. This is an insulating resin sheet whose glass transfer temperature exceeds 100.degree. C. In the present semiconductor module, the cheap metal base 15 with no insulating layer and laminated conductors formed thereon is used in place of the insulated metal circuit board 19 requiring a complex manufacturing process such as etching for forming the laminated conductors 14. The insulating adhesive sheet 18 and lead frames 13 fabricated by punching with a press or the like are adhered to the metal base 15 to thereby achieve the function equivalent to the insulated metal circuit board 19. Therefore, the cost-down of each member can be expected and the area of each pad can be reduced owing to the thinning of each wire or interconnection and a reduction in the number of the metal wires 16, thus making it possible to bring the module into less size and cost.
In the case of the structure having the resin-molded outer package 33, the insulated metal circuit board 19 and the printed circuit board 21a shown in FIG. 2, the area of the insulated metal circuit board 19 must be reduced to achieve a further reduction in the size and cost of the power converter. It is however necessary to take measures for reducing the wiring width upon reducing a board. In this case, the wiring thickness of each laminated conductor 14 needs to be increased for the purpose of maintaining current capacity. However, the range of a fabricable thickness of each laminated conductor 14 on the insulated metal circuit board 19 is relatively thin as normally in the case of a range from 70 .mu.m to 200 .mu.m. Therefore, the current capacity per unit wiring width is reduced so that it becomes difficult to thin the wiring width. This point of view is one factor for determining the limit of a size reduction. While the electrical connections between the main terminals 17 corresponding to the input/output unit of the power converter, the respective power semiconductor elements 11a and 11b, the laminated conductors 14 on the insulated metal circuit board 19, etc. are made by wiring bonding using the metal wires 16, a plurality of the metal wires 16 consistent with the current capacity are required, thus resulting in an increase in the area of each pad used as the connecting portion for each metal wire 16 and interfering with a reduction in size and cost.
On the other hand, the structure (transfer mold) in which the power semiconductor elements 11a and 11b and printed circuit board 21a are adhered to the metal base 15 with the insulating adhesive sheet 18 and the lead frames 13 interposed therebetween as shown in FIG. 3 needs to mold at a time. This involves the following problems:
(1) Since the resin sealing agent 32 covers the surfaces of the power semiconductor elements 11a and 11b, the integrated circuit part 23, the metal wires 16, etc., they need to be integrally molded by using a resin high in flowability. In this case, a mold for minimizing peripheral tolerance of each output terminal is required to prevent the resin from being injected round. Thus, when compared with the molded package structure shown in FIG. 2, the mold increases in cost and hence the present structure was unfit for the application to small-batch products.
(2) Stress is developed due to the expansion and shrinkage of a resin used for a molding, and a bend of a metal base increases substantially in proportion to the molding size. A limitation to its outer size is made to control its bend, so that a free system configuration could not be constructed.
(3) When one attempts to set the direction of each electrode withdrawn to the outside of a semiconductor module in the direction orthogonal to the metal base upon molding a semiconductor device, the mold becomes complex and very high in cost. Therefore, terminals are normally led out so as to become parallel to the metal base. However, in the structure wherein such terminals are guided so as to become parallel to the metal base, a large restriction is made to a connection to an external circuit when a power converter or the like is taken into consideration, so that a system including the semiconductor device cannot be reduced in size. While an external radiation fin is attached to the lower surface of the metal base to cool a semiconductor module, the guiding direction of each terminal shown in FIG. 3 has a bearing even on a problem about the distance for insulation between the external radiation fin, each main terminal and each control terminal, and the vertical height of each electrode withdrawn portion with respect to the metal base cannot be set to the insulation distance or less, whereby the height should inevitably be increased. This brings about an increase in the amount of a resin for molding and the bend of the metal base due to the expansion and shrinkage of the resin increases, thus causing a problem similar to the above (2).
(4) Since the integral formation of parts having bumps and dips or irregularities as in the case of an isolation transformer and an electrolytic capacitor necessary to construct the power converter together with lead frames needs to completely cover all the parts, a large quantity of resin is required and hence the cost thereof rises steeply so that a cheap device cannot be constructed.